tests we consider 75KΩ to be the default R0 for the CO sensors and 10KΩ to be the default R0 for NO2. We can calculate ppm as RS/R0. Example: RS = 641KΩ will around 8.3 PPM http://www.aqmd.gov/docs/default-source/aq-spec/field-evaluations/smart- citizen-kit---field-evaluation.pdf?sfvrsn=2 PPM = RS/R0